2017: fix exporting-go.pdf, slide 26

The readNode function must return a node.

Change-Id: I37926323c5e7d6887c950d948419913bd32d8547
Reviewed-on: https://go-review.googlesource.com/44990
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

2017/exporting-go.pdf

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